(Bloomberg) — When Paris suffered a heat wave in 2003 that killed an estimated 15,000 people across France, one oasis of cool air remained off-limits to access: the city’s roughly 400 public parks, which were temporarily closed due to the danger of falling tree branches.

“It basically took away the one little green space that people might have had to find a slightly cooler climate,” says Richard C. Keller, a professor of medical history at the University of Wisconsin-Madison and author of Fatal Isolation: The Devastating Paris Heat Wave of 2003.

When trees lose their branches, it is usually associated with severe weather such as tornadoes, hurricanes and thunderstorms. However, the risk is little known outside of arborist circles: mature and apparently healthy trees can suddenly lose branches in summer weather. This phenomenon is called sudden branch failure, sudden branch drop or summer branch drop.

While reports of injuries from sudden tree branch failure are rare, a woman was killed by a falling branch in a Washington DC park last month. The incident prompted at least one local tree service to send out an email warning the public of the risk.

Why apparently healthy trees suddenly lose their branches is a bit of a mystery. “There is no scientific consensus on why this happens,” says Spencer Campbell, director of the plant clinic at the Morton Arboretum in Lisle, Illinois. “It’s difficult to predict.”

But at least one theory links summer branch drop to extreme heat — especially when a hot summer is preceded by a wet spring. Wet springs allow for lush tree growth, which can lead to water stress in hotter summer temperatures. Trees may shed branches as a form of self-pruning.

Another theory is that extreme winter weather—from cold temperatures to the pressure that snow puts on branches—causes trees to weaken internally, causing them to push over the edge under the pressure of new branch growth in the spring.

Summer branch drop was once considered a phenomenon of hot climates and was observed in areas such as Australia, South Africa and the southwestern United States, where snowfall is not usually high. However, since the 1970s, arborists have also reported this in areas with a wider range of climates, including Illinois, New York and the United Kingdom.

Trees that suddenly lose branches may also be diseased, even if it’s not visible. (The tree in Washington was examined, pruned and found healthy in 2022, according to news reports.) “Professional arborists are trained to identify trunk rot, canker (and other) physical symptoms,” Spencer says, adding that “it’s harder to diagnose whether it’s an internal crack or a weak spot.”

The most likely conclusion is that the sudden loss of branches is due to a confluence of circumstances: trees that are on the verge of collapse and conditions such as extreme heat that push the trees to their breaking point.

This was the conclusion of a 2023 case study that looked at the sudden shedding of branches in four cities in Portugal. The researchers concluded that branch shedding in summer was related to the internal degradation of the trees, but noted that the degradation itself could also be related to environmental conditions.

“I think for many trees in our landscape, it’s not just one thing, but many stress events,” Campbell says. Possible causes range from unusually warm evenings to cold snaps, fungal diseases and insect damage, exacerbated by rising temperatures.

Whatever the cause, Campbell says there’s no need to worry too much if you’re standing under trees, even in the summer. “It’s extremely rare for something like this to happen,” he says. “It does happen, but most of the time it happens without anyone noticing.”

Read more about the coverage of the heat week:

Listen to Zero: What extreme heat means for your health

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